1. Field of the Invention
The invention relates to fluid-operated brake actuators for vehicles and more particularly to service and spring brake actuators combined in tandem and having an improved spring brake pressure plate.
2. State of the Prior Art
An air brake system for a vehicle such as a bus, truck or the like typically includes a brake shoe and drum assembly which is actuated by means of an actuator assembly operated by the selective application of a fluid such as compressed air. Conventional air brake actuators have both a service brake actuator for actuating the brakes under normal driving conditions by the application of compressed air and an emergency or spring brake actuator for actuating the brakes when the compressed air is released. The emergency brake actuator includes a strong compression spring which forces application of the brake when air is released. This actuator is often referred to as the spring brake. Typically, the spring brake actuator is disposed in tandem with the service brake actuator.
The service brake actuator is typically divided into two chambers by a diaphragm. Depressing the brake pedal during normal driving operation introduces into one of the chambers of the service brake actuator compressed air which, acting against the diaphragm, causes a service brake push rod in the opposite chamber to be extended and the brakes to be applied with an application force proportional to the air pressure in the service brake actuator.
In tandem actuator assemblies, a spring brake push rod typically extends from a chamber in the spring brake portion, through an aperture in a wall separating the spring brake actuator from the service brake actuator, and into a chamber in the service brake portion. Because at least one of the adjoining chambers is usually pressurized, a seal is provided at the aperture around the push rod comprising one or more O-rings positioned in annular channels in the wall around the aperture.
The spring brake actuator is typically divided into two chambers, a lower chamber and an upper chamber, separated by a rubber diaphragm and pressure plate, with the spring in the upper chamber acting between an end wall of the spring brake housing and a pressure plate. When full pressure is applied to the lower chamber, air pressure acting against the diaphragm and pressure plate compresses the spring. In many applications, the spring brake actuator rod is held in a retracted position by a relatively small return spring. In some actuators, the spring brake actuator rod is integral with the pressure plate and held in a retracted position by the air pressure. In both designs, the spring brake actuator rod thus does not affect the normal operation of the brake.
In the event of a loss of air pressure or an intentional exhaustion of air from the spring brake actuator, the brake will be mechanically activated by the force of the strong compression spring acting on the spring brake actuator rod which, in turn, acts upon the service brake push rod to apply the brakes. Thus, the spring brake portion serves both as a parking brake and an emergency brake.
The volume of air in the lower chamber of the spring brake actuator is related to the response time of the spring brake. The response of the spring brake is the time it takes the spring to force the spring brake push rod the necessary distance to activate the brakes after air pressure to the lower chamber is lost. For a given air pressure, the greater the volume of the lower chamber, the longer the response time for the application of the spring brake, after the air is released. Therefore, it is desirable to reduce the volume of air in the lower chamber of the spring brake to decrease the time for the application of the spring brake after the air is released. However, the volume of the lower chamber cannot be reduced by merely reducing the size of the spring brake actuator because the spring must be of sufficient size to stop the vehicle and the spring must travel a sufficient distance so the complementary travel of the push rod is sufficient to actuate the brake.